The term "molecular sieve" refers to a wide variety of positive ion containing crystalline materials of both natural and synthetic varieties which exhibit the property of acting as sieves on a molecular scale. A major class of molecular sieves are crystalline aluminosilicates, although other crystalline materials are included in the broad definition. Examples of such other crystalline materials include coal, special active carbons, porous glass, microporous beryllium oxide powders, and layer silicates modified by exchange with organic cations. See, D. W. Breck, "Zeolite Molecular Sieves: Structure, Chemistry, and Use", John Wiley & Sons, 1974.
Zeolites are crystalline, hydrated, framework aluminosilicates which are based on a three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygens.
Zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein, x is generally equal to or greater than 2 since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation, M, and water molecules. The cations may be mobile and exchangeable, to varying degrees, by other cations. Intracrystalline zeolitic water in many zeolites is removed continuously and reversibly. In many other zeolites, mineral and synthetic, cation exchange or dehydration may produce structural changes in the framework. Ammonium and alkylammonium cations may be incorporated in synthetic zeolites, e.g., NH.sub.4, CH.sub.3 NH.sub.3, (CH.sub.3).sub.2 NH.sub.2, (CH.sub.3).sub.3 NH, and (CH.sub.3).sub.4 N. In some synthetic zeolites, aluminum cations may be substituted by gallium ions and silicon ions by germanium or phosphorus ions. The latter necessitates a modification of the structural formula.
The structural formula of a zeolite is best expressed for the crystallographic unit cell as: M.sub.x/n [(AlO.sub.2).sub.x (SiO.sub.2).sub.y ].wH.sub.2 O where M is the cation of valence n, w is the number of water molecules and the ratio y/x usually has values of 1-100 depending upon the structure. The sum (x+y) is the total number of tetrahedra in the unit cell. The complex within the [] represents the framework composition.
The zeolites described in the patent literature and published journals are designated by letters or other convenient symbols. Exemplary of these materials are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), and Zeolite ZSM-12 (U.S. Pat. No. 3,832,449).
Although there are 34 species of zeolite minerals and about 100 types of synthetic zeolites, only a few have been found to have practical significance. Many of the zeolites, after dehydration, are permeated by very small channel systems which are not interpenetrating and which may contain serious diffusion blocks. In other cases dehydration irreversibly disturbs the framework structure and the positions of metal cations, so that the structure partially collapses and dehydration is not completely reversible. To be efficiently used as a molecular sieve, the structure of the zeolite after complete dehydration must remain intact.
There has been considerable interest in developing metallosilicates other than aluminosilicates which exhibit molecular sieve characteristics. For example, U.S. Pat. Nos. 3,329,480 and 3,329,481 disclose crystalline zircano-silicates and titano-silicates, respectively. U.S. Pat. No. 3,329,384 discloses group IV-B metallosilicates. U.S. Pat. Nos. 4,208,305, 4,238,315 and 4,337,176 disclose iron silicates. U.S. Pat. No. 4,329,328 discloses zinco-, stanno-, and titano-silicates. European patent applications 0 038 682 and 0 044 740 disclose cobalt silicates. European patent application 0 050 525 discloses nickel silicate.
U.K. patent application GB 2,024,790 A discloses a silica-based material which has been modified with one or more elements which have entered the crystalline lattice of the silica in place of silicon atoms of the silica or in the form of salts of bisilicic or polysilicic acids. The elements identified as being suitable for making such silica-based materials are chromium, beryllium, titanium, vanadium, manganese, iron, cobalt, zinc, zirconium, rhodium, silver, tin, antimony and boron.
U.S. Pat. No. 4,299,808 discloses chromosilicates formed by reacting an aqueous mixture of an oxide of silicon, a compound of chromium, a hydroxide of an alkali or an alkaline earth metal, and an alkylammonium cation or a precursor of an alkylammonium cation.
U.S. Pat. No. 3,769,386, 4,192,778 and 4,339,354 relates to rare earth metal containing silicates. U.S. Pat. No. 3,769,386 discloses zeolitic alumino-metallosilicates crystallized from an aqueous reaction mixture containing Na.sub.2 O, SiO.sub.2, Al.sub.2 O.sub.3 and R.sub.2/n wherein R is Mg, Ca, Y, Fe, Co, Ni or a rare earth metal and n is the valence of R. U.S. Pat. No. 4,192,778 discloses rare earth exchanged zeolites of the faujasite type in which the equivalent of Na is less than 0.1 and the rare earth is at least 0.9 equivalents per gram atom of aluminum. U.S. Pat. No. 4,339,354 discloses a catalyst comprising a crystalline aluminosilicate such as zeolite Y, an inorganic matrix, and discrete particles of alumina, the catalyst having specified alkali metal and rare earth metal contents.
The term "silica molecular sieve" refers to molecular sieves of silica that do not contain framework aluminum or other framework metal, except for such aluminum or other metal that may be present as impurities. By definition these molecular sieves are not zeolites, aluminosilicates or metallosilicates. An example of a silica molecular sieve is silicalite which topologically resembles ZSM-5 and contains the same type of building unit. After synthesis and calcination, the inert cell of 96 SiO.sub.2 tetrahedra has a pore volume of 0.32 cm.sup.3 /cm.sup.3. The structure of silicalite consists of sheets of hexagonal SiO.sub.4 rings, three layers wide extending in one direction, which are cross-linked by two SiO.sub.4 units. U.S. Pat. No. 4,061,724 discloses crystalline silica compositions.
Synthesis directing agents are agents which when added to a reaction mixture effect a different crystalline structure than that which would occur but for the addition of such agent, or result in the formation of a crystalline structure wherein an amorphous mixture would otherwise remain. Examples of conventional agents that are currently in use include ammonium and tetrapropyl ammonium ions, and alkyl phosphonium and alkyl arsonium ions. See, L. D. Rollman, "Templates in Zeolite Crystallization", Adv. Chem. Ser., 173, 387 (1979).
U.S. Pat. No. 4,018,870 discloses the use of certain nitrogen containing dyes including methyl violet and methylene blue in the synthesis of zeolites. This patent indicates that these dyes are used to suppress the formation of unwanted zeolites during the manufacture of more desirable products.
It would be advantageous to employ nitrogen containing organic dyes as synthesis directing agents in the synthesis of silica molecular sieves.